The present invention relates to a control circuit for switching inductive loads which can be monolithically integrated and used in high-speed printing equipment to drive the electromagnets of the printing elements and in chopper power supply systems.
A control circuit for switching in accordance with the present invention comprises a final power transistor which is connected in series with an inductive load, the pair being connected between two terminals of a D.C. voltage supply. The transistor is driven alternately, by means of a base control signal, from a first state of high voltage and low current to a second state of low voltage and high current.
In the first state, the transistor is virtually an open circuit between the emitter and collector terminals, (i.e.--the open or "off" state), and in the second state, the transistor is essentially a short-circuit between the emitter and collector terminals, (i.e.--the conductive or "on" state), thereby respectively preventing or allowing a current to flow through the inductive load.
The operating mode of the transistor which is closest to the operation of an ideal switch is that in which the transistor operates at saturation when in its conductive or on state and is turned off in its open or off state. In this case, the maximum possible switching frequency of the final power transistor is essentially limited, during the transition phase from the saturation state to the off state, by the effects of the charge storage of the transistor which has occurred during the conductive state.
As is known, power transistors generally have a thick collector region having a high resistivity, since it is necessary for the transistor to withstand a high reverse voltage.
This region of the collector has a turn off transient operation constituted by a first phase in which the transistor continues to be saturated, a second phase of "semi-saturation" in which the collector-emitter voltage starts to rise, but the collector current remains constant, and a final phase in which the collector-emitter voltage rises rapidly and the collector current reduces to zero.
The phase of "semi-saturation" is, in particular, that phase in which the transistor dissipates the most energy.
Therefore, a reduction in the turn-off time would be advantageous both for increasing the maximum possible switching frequency and for improving the efficiency of the control circuit from the point of view of energy consumption, thereby reducing the times in which the operation of the final power transistor is different from that of an ideal switch.
A known circuit solution of the abovenoted problem is to connect a low impedance circuit means to the base of the final power transistor. When the transistor is turned off, this circuit means allows a rapid removal of the charge carriers stored in the transistor. As an example, such a circuit means comprises a transistor of suitable dimensions, connected to the base of the final transistor, which functions in phase opposition with respect to the latter, and removes the stored charge in order to accelerate the turn-off of the final transistor.
Circuit solutions of this prior art type are described, for example, in the United Kingdom patent application No. 2,053,606 and U.S. Pat. No. 4,092,551.
However, even the charge removal or speed up transistor has its own turn-off time, although small. Thus, since the charge removal transistor operates in a conduction state which is opposite to that of the final power transistor, the charge removal transistor determines the time delay for again turning on the final power transistor, and such a time delay is not negligable if the maximum switching speed is desired. On the other hand, not even the useless absorption of supply current used to keep the charge removal transistor active when the final transistor is already completely turned off can be entirely disregarded.
When the base control signal is transmitted to the final power transistor by means of a second transistor coupled thereto, the operating speed of the control circuit also depends upon the maximum switching speed of this second transistor, which in turn depends upon the effects already noted of storing a charge in the base thereof if the second transistor operates at saturation in its conductive state.
In this case, the speed limitations which derive therefrom can be considerable, especially for control circuits which are monolithically integrated and comprise a PNP type transistor for driving the final power transistor (which is generally of an NPN type for reasons relating to the integration thereof, as is well known to persons skilled in the semiconductor art ). Even when the final output transistor is in its conductive state, it operates in the active zone of its operating range, since such integrated PNP type transistors inherently have a longer turn-off time than that of NPN transistors.